1. Field of the Invention
The present invention relates to a methods for producing modified milk. The present invention further relates to methods for producing a dairy product using such a modified milk.
2. Discussion of the Background
Raw milk (milk fresh from a cow) lends our everyday meals affluence by being processed into various kinds of dairy products, as well as for drinking as a milk, such as an ice cream, yoghurt, cheese, powdered milk, lactic acid bacteria beverage, and the like made from milk or butter made from cream separated from raw milk.
One of the most efficient indicators for determining “tastiness” of such dairy products is dense and creamy texture of milk fat and it is said that the texture has a close relation with fat content of the dairy products. On the other hand, recently, an attention to easiness to purchase such as a “low-price” product and the like, or to health for consumers such as a “low-calorie” product and the like, is increasing as a factor for determining a commercial value. Although various kinds of dairy products containing less milk fat content, such as no fat products or low fat products, are developed according to the increasing demands, the taste of these products is still plain, watery and less milk flavor, and therefore consumers were not satisfied. Thus it is required to develop tasty dairy products that can satisfy consumers' demands even when milk fat content is low or without increasing milk fat content.
By the way, nowadays enzymes are utilized in various fields in a food industry. The merits of using enzymes are given that an enzyme acts only on specified material under mild conditions and that an influence on taste is small. Developments and improvements of foods or improvements of manufacturing processes are carried out for various purposes by utilizing such superior characteristics of enzymes.
Among them, a protein deamidating enzyme is attracting our attention as a new enzyme for giving dense and creamy texture, which can be obtained when milk fat is added, to dairy products. The enzyme is a catalytic enzyme that facilitates a deamidating reaction by acting on a glutamine residue in a protein and it is also referred to as a protein glutaminase (hereinafter designated as “PG”). The glutamine residue in a protein is converted into glutamic acid residue and produces carboxylic group, and therefore increasing of negative charge, increasing of electrostatic repulsion, decreasing of isoelectric point, increasing of hydrating power, and the like occur in the protein. It is known that, as a result, improvement of various kinds of functional characteristics can be obtained such as increasing of solubility and dispersing ability in water, increasing of emulsificating ability and stability, and the like, of the protein (see, Japanese Patent Kokai Publication No. JP-P2000-50887A; Japanese Patent Kokai Publication No. JP-P2001-218590A; Yamaguchi et al., Appl. Environ. Microbiol., vol. 66, pp. 3337-3343 (2000); Yamaguchi et al., Eur. J. Biochem., vol. 268 pp. 1410-1421 (2001); Matsumura et al., J. Agric. Food Chem., (2001), vol. 49, pp. 5999-6005; and Comprehensive Encyclopedia of Milk, edited by Kunio Yamauchi, Asakura Shoten, p. 60, 1992, which are incorporated herein by reference in their entireties). It is also known that PG having such characteristics improves the smooth texture of dairy products such as yoghurt or cheese by using PG for such products (see, Comprehensive Encyclopedia of Milk, edited by Kunio Yamauchi, Asakura Shoten, p. 60, 1992, which is incorporated herein by reference in its entirety). Thus it is expected that PG may provide the dense and creamy texture that the milk inherently possesses even when the fat content in a dairy product was lowered.
Milk proteins contained in milk, the content of which is 3.0 to 3.5%, are classified into two large groups, one is casein and the other is whey protein. Casein amounts to about 80% of the total protein and whey protein amounts to about 20% of the total protein. Casein is made up of four components of αs1-casein, αs2-casein, β-casein and κ-casein, and whey protein is made up of α-lactalbumin and β-lactglobulin.
At present, some reports can be seen that relate to PG action on such milk proteins. Yamaguchi et al., Appl. Environ. Microbiol., vol. 66, pp. 3337-3343 (2000), which is incorporated herein by reference in its entirety, discloses that the reactivity of PG with α- and β-casein is high and the reactivity of PG with α-lactalbumin and β-lactglobulin is low. Matsumura et al., J. Agric. Food Chem. (2001), vol. 49, pp. 5999-6005 and WO2002/068671, which are incorporated herein by reference in their entireties, disclose that the reactivity of PG with α-lactalbumin in the molten globule state, that is, during intermediate state in denaturation process of a protein, increases as compared with non-denaturation state.
It is not too much to say that the reactivity of PG with milk depends on the reactivity of casein, which amounts to the most part of milk, rather than the reactivity of whey protein. The most part of casein is dispersed in milk in the form of colloidal particles that are called as casein micelle whose diameter is 0.05 to 0.3 microns and mean diameter is about 0.1 microns. In addition, it is known that thermally sensitive whey protein interacts with the casein micelle (especially κ-casein that covers a surface of the casein micelle) by heating (see, Comprehensive Encyclopedia of Milk, edited by Kunio Yamauchi, Asakura Shoten, p. 60, 1992, which is incorporated herein by reference in its entirety).
However, the disclosures of Yamaguchi et al., Appl. Environ. Microbiol., vol. 66, pp. 3337-3343 (2000); Matsumura et al., J. Agric. Food Chem. (2001), vol. 49, pp. 5999-6005; and WO2002/068671, which are incorporated herein by reference in their entireties, are results of research at an experimental reagent level and nothing is referred to for reactivity of the whole milk as a raw material of dairy products. It is speculated that the reactivity of PG in a complicated milk system as disclosed in Comprehensive Encyclopedia of Milk, edited by Kunio Yamauchi, Asakura Shoten, p. 60, 1992, which is incorporated herein by reference in its entirety will differ from that in a single component of milk protein; however, the reactivity of PG in milk has not been reported.